The present invention relates to magnetic polishing fluids for polishing ceramic or optical materials.
Chemical-mechanical polishing is a commonly used technique for planarization of bare silicon, silica and other ceramic wafers. In chemical-mechanical polishing, the wafers are held against a rotating polishing pad wetted by a slurry consisting of colloidal abrasive particles (silicon dioxide, alumina, cerium oxide, etc.) and specific additives to enhance the rate and quality of polishing. When silica is used, a high pH is required. An oxide or hydroxide surface layer forms on the wafer and the reaction products are removed by mechanical action of the pad and the colloidal silica slurry. Similar processes occur when other abrasives are used.
The bending and stresses of the rotating polishing pad strongly influence the quality of planarization. The adverse effects of a solid polishing pad can be avoided, or at least mitigated, by using a magnetic polishing fluid which forms a plasticized mass in an a inhomogeneous magnetic field.
Magnetorheological fluids (MRFs) for polishing are known in the art. With MRFs, the polishing pad common to conventional polishing is dispensed with entirely.
The viscosity of magnetorheological fluids increases in a magnetic field and the fluids, which contain magnetic particles, acquire the characteristics of a plasticized solid, forming an abrasive mass with a viscosity and elasticity suitable for polishing.
Kordonsky et.al., U.S. Pat. No. 5,449,313, describes a magnetorheological fluid which comprises a plurality of magnetic particles (carbonyl iron particles), abrasive particles (cerium oxide), a stabilizer (fumed silica), and a carrying fluid consisting of a mixture of water and glycerin.
Kordonsky et.al., U.S. Pat. No. 5,525,249, describes a magnetorheological fluid comprising magnetosolid and magnetosoft particles, a stabilizer (fumed silica), and a carrying fluid comprising an aromatic alcohol, vinyl ester, and an organic solvent or diluent carrier such as kerosene,
Jacobs et al. U.S. Pat. No. 5,795,212 describes a magnetic fluid which uses iron particles together with CeO2 particles, the latter serving as an abrasive polishing agent.
Jacobs et al,. U.S. Pat. No. 5,804,095, describes a magnetorheological polishing fluid which comprises among other components magnetic particles, a CeO2 abrasive polishing agent, and Na2CO3.
Kato et al. 4,91,466 recites the use of a magnetic fluid comprising colloidal iron particles and abrasive particles with a size of about 40 xcexcm.
Polishing machines used with prior art MRF compositions cause the MRFs to acquire viscous, plasticized properties known as Bingham properties, when the fluids are subject to the influence of magnetic forces. When in a magnetic field, the MRF is hard enough to be used as a polishing tool. However, these polishing fluids reach their fully developed Bingham state only at the outset of the polishing process. Once the substrate being polished begins to move relative to the MRF, the shear stress of the abrasion process causes the MRF to lose its Bingham properties and its plasticized characteristics. The MRF remains viscous throughout the process but returns to a liquid, non-plasticized state almost immediately after abrasion begins, thereby reducing polishing efficiency.
The magnetorheological fluids in the above-cited compositions usually do not provide a chemical mechanism for enhancing the efficiency of the polishing process. In addition, these fluids generally contain magnetic and/or abrasive particles with sizes in excess of 1 xcexcm. These large size particles, and especially their agglomerates, introduce defects, scratches and non-uniformities onto the surface being treated.
Finally, because of the large size magnetic or abrasive particles used, these fluids have a tendency to settle due to gravitational forces. Because of their size, the abrasive particles act as agglomeration centers, contributing to the aggregation of magnetic particles and to the instability of the magnetic fluid. Any use of such fluids requires prior agitation.
The present invention discusses magnetic polishing fluid compositions and their methods of preparation. These compositions can be used to polish many different types of materials employed in the semiconductor, optical and microelectronic industries.
The present invention teaches various magnetic polishing fluid compositions. One embodiment of such a composition includes colloidal size magnetic particles where the mean particle size ranges from about 0.01 xcexcm to about 0.15 xcexcm, colloidal size polishing particles where the mean particle so ranges from about 0.01 xcexcm to about 0.1 xcexcm, at least one stabilizer and a carrying fluid where the pH of said composition is greater than 9 to about 12.
Another magnetic polishing fluid composition embodiment comprises colloidal and non-colloidal magnetic particles where the non-colloidal magnetic particles have a mean particle size ranging from about 0.15 xcexcm to about 3.0 xcexcm and the colloidal magnetic particles have a mean particle size from about 0.01 xcexcm to about 0.15 xcexcm. The embodiment includes colloidal size polishing particles which have a mean particle size ranging from about 0.01 xcexcm to about 0.1 xcexcm and at least one stabilizer for stabilizing the colloidal size particles. Finally, the composition contains an additive for adjusting viscosity, and a fluid wherein the pH of said composition is greater than 9 to about 12.
A third magnetic polishing fluid embodiment comprises magnetic particles with a mean particle size ranging from about 0.15 xcexcm to about 3.0 xcexcm, colloidal size polishing particles with a mean particle size ranging from about 0.01 xcexcm to about 0.1 xcexcm, at least one stabilizer for stabilizing the colloidal particles, an additive for adjusting viscosity, and a carrying fluid where the pH of said composition is greater tan 9 to about 12.
The invention teaches a method for preparing a composition comprising the steps of preparing a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid, preparing a colloidal size polishing particles dispersion in a carrying fluid, mixing said colloidal size magnetic particle dispersion and said colloidal size polishing particle dispersion to form a mixture, and adjusting the pH of said mixture to greater than 9 to about 12, if the mixture is not already at that pH.
Another method for preparing certain compositions of the invention includes the steps of preparing a non-colloidal size magnetic particle dispersion by adding the particles to a mixture of a viscosity additive in a carrying fluid, preparing a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid, preparing a colloidal size polishing particle dispersion in a carrying fluid, mixing the colloidal size magnetic particle dispersion and the colloidal size polishing particle dispersion to form an intermediate mixture, mixing the intermediate mixture with the non-colloidal size magnetic particle dispersion, and adjusting the pH of the composition to a pH in the range of from greater than 9 to about 12, if the final composition is not already at that pH.
The invention also teaches several specific compositions one of which includes colloidal size magnetic particles comprised of colloidal size magnetite (Fe3O4) particles having a weight percent content ranging from about 20 to about 55 wt % of the final composition, colloidal size polishing particles comprised of colloidal size silica (SiO2) particles having a weight percent content ranging from about 1 to about 20 wt % of the composition, at least one stabilizer comprised of tetramethylammonium hydroxide having a weight percent content ranging from about 0.05 to about 25 wt. % of the composition and a carrying fluid, adjusted to a pH greater than 9 to about 12, comprising any remaining wt % of said composition. The carrying fluid comprises the remaining wt % of the composition.
Another composition taught includes non-colloidal size magnetic particles selected from carbonyl iron or magnetic particles, the particles having a weight percent content from about 21 to about 38.5 wt %. The composition also includes colloidal size magnetic particles comprising colloidal size magnetite particles having a weight percent content form about 3 to about 7.5 wt % and colloidal size polishing particles comprised of colloidal size silica particles having a weight percent content from about 1 to about 10 wt %. It also includes a stabilizer, tetramethylammonium hydroxide having a weight percent content from about 0.5 to about 10 wt. % and a viscosity adjusting additive comprising polyethylene glycol having a weight percent content from about 0.1 to about 5 wt. %. Finally, the composition is completed with a carrying fluid adjusted to a pH greater than 9 to about 12. The carrying fluid comprises the remaining wt % of the composition.
A third composition taught by the present invention includes magnetic particles which are carbonyl iron particles. These iron particles have a weight percent content in the composition from about 20 to about 55 wt. %. The composition also includes colloidal size polishing particles comprised of colloidal size silica particles having a weight percent content in the composition from about 1 to about 20 wt. % and at least one stabilizer comprised of tetramethylammonium hydroxide having a weight percent content in the final composition ranging from about 0.5 to about 25 wt. %. The composition also contains an additive for adjusting viscosity, specifically polyethylene glycol, which has a weight percent content ranging from about 0.1 to about 10 wt. %. Finally, a carrying fluid, adjusted to a pH greater than 9 to about 12 comprises the remaining wt % of the composition. The carrying fluid comprises the remaining wt % of the composition.
The present invention relates to the composition of various magnetic polishing fluids (MPFs) which can be used to polish different ceramic substrates, such as silica, alumina, cerium oxide, and glass. It is particularly advantageous as a composition for polishing silica (SiO2). By varying the composition of these MPFs, the rate and quality of polishing can be controlled. The MPFs of the present invention polish at lower viscosities than the MRFs cited above, thereby also producing better quality surfaces. It is believed that the magnetic particle polishing process of this invention is abetted by chemical processes similar to those which occur in chemical-mechanical polishing.
A magnetic polishing fluid (MPF) in a magnetic field, although forming a plasticized mass when the field is activated, retains or reverts to the properties of a liquid during the polishing process. When in its plasticized state the mass possesses Bingham properties.
A magnetic polishing fluid comprised of magnetic particles, responds to a magnetic field gradient by increasing its viscosity and changing into a plasticized mass. Any polishing particles contained in the fluid are trapped in the plasticized mass and pushed out of the fluid in a direction perpendicular to the gradient.
This mass is an effective polishing tool when it comes into contact with a moving work surface. Upon contacting the polishing surface, and under the shear stress of the abrasion process, the plasticized mass reverts back to an almost liquid state.
In addition, when a polishing solution contains magnetic particles, particularly magnetic particles of non-colloidal size, the particles interact and form dendritic structures when plasticized under the influence of magnetic fields. These plasticized structures contain pores. A liquid media containing a polishing slurry comprising colloidal abrasive particles can fill the pores in the plasticized structure. The polishing rate is enhanced when the pores in the plasticized polishing mass are filled in this manner. The liquid containing the abrasive particles rises to the surface of the plasticized mass through capillary action. At the surface, the polishing particles are pressed by the polishing machine against the work surface.
The magnetic polishing fluid compositions of the present invention comprise, at a minimum, at least one type of magnetic particles, colloidal size abrasive particles, a stabilizer, and a carrying fluid. In one embodiment, the magnetic particles are of colloidal size and first dispersed in a stabilizer in a carrying fluid before mixing with colloidal abrasive particles. In another embodiment, two sizes of magnetic particles are used, one colloidal and one non-colloidal. Each size of magnetic particle is dispersed separately in its own carrying fluid before the two dispersions are mixed.
For purposes of the present invention, colloidal size magnetic particles are deemed to be particles with mean particle sizes of about 0.15 xcexcm or less, while colloidal size polishing particles are deemed to be particles with mean particle sizes of about 0.1 xcexcm or less. For purposes of the present invention, a ceramic is any non-biological material which has neither clearly defined metallic nor non-metallic characteristics and properties. It includes among other things classical ceramics, glasses and metalloids. Among other materials, it includes many oxides, nitrides, borides, and phosphides. Many of the materials used in the optical, microelectronic and semiconductor industries would be included in the definition of ceramics as used herein.
Certain embodiments of the invention may require a viscosity additive to control viscosity and/or a pH additive to adjust pH. Additives to increase the rate and enhance the quality of the polishing process can be added to the composition, as needed. In all the embodiments discussed, there is little agglomeration or sedimentation in the fluid over a period of one year at room temperature.
The magnetic particles used in these compositions can be chosen from many different ferrous and non-ferrous magnetic materials. These include, but are not limited to, ferromagnetic metals such as Fe, Ni, and Co, their alloys, ferrites, chromium oxides such as CrO2, iron oxides such as Fe3O4, and cobalt oxides such as Co3O4, Carbonyl iron or magnetite (Fe3O4), particularly the latter, are the preferred sources for non-colloidal size magnetic particles. The preferred mean particle size range for non-colloidal magnetic particles is generally about 0.15-3.0 xcexcm, and even more preferably 0.5-1.0 xcexcm.
Capillary action is needed to transport the liquid containing the abrasive colloidal particles to the surface of the plasticized magnetic particle mass. The magnetic particles form quasi-columns with the pathways between them thought of as capillary tubes. The average diameter of the capillary is determined by magnetic particle size. The capillary must be narrow enough to generate a large capillary force but not too narrow to be clogged by abrasive particles. A rough calculation based on these principles, assuming abrasive particles with diameters of about 0.1 micron, can establish 3 microns as an approximate upper bound for the magnetic particles that should be used.
Colloidal magnetite (Fe3O4), the preparation of which is described below, is the preferred source of colloidal magnetic particles. The particles are formed from a mixed ferrous-ferric chloride solution to which either ammonium hydroxide, NaOH or urea is added. Methods for preparing colloidal magnetite other than the one disclosed herein below can also be used, such as the method of Massarat (IEEE Transactions of Magnetics, Vol. Mag 17 (#2) March 1981). Colloidal magnetic dispersions are also commercially available from manufacturers such as Ferrofluids, Inc. The preferred mean particle size range for colloidal magnetic particles is generally about 0.01-0.15 xcexcm, preferably 0.03-0.05 xcexcm.
Colloidal size polishing particles can be chosen from many types of oxides, such as xcex1-Fe2O3, ThO2, ZrO2, SnO2, CeO2, SiO2 or ZnO. Colloidal size particles of carbides e.g. SiC, nitrides e.g. Si3N4 and polycrystalline diamond can also be used as abrasives. The colloidal polishing particles, preferably silica (SiO2), generally have a mean particle size range of about 0.01-0.1 xcexcm. An even more preferred mean particle size range for the polishing particles is 0.03-0.05 xcexcm. In the case of diamond, nanometer size particles can be used.
A stabilizing additive for the polishing agent can be chosen from many different non-ionic, anionic and cationic stabilizers, particularly from the group of alkylated ammonium hydroxides. In a preferred embodiment of the invention the stabilizing agent is tetramethylammonium hydroxide (TMAH), which also helps adjust pH. Surfactants that can be used as stabilizers are polydispersed nonylphenol, sodium polyethoxylate, sodium dodecylsulfonate, decylmethylsulfoxide and sodium lignosulfonates. The same stabilizer is usually, but not necessarily used to stabilize both colloidal size magnetic and colloidal size polishing particles. Use of low molecular weight stabilizers are preferred since high molecular weight stabilizers, and in particular polymeric surfactants, decrease the rate of polishing, sometimes by as much as a factor of four or five.
All of the magnetic polishing fluids require a carrying fluid. The preferred carrying fluid in all embodiments is water, and even more preferred is de-ionized water. Water is a preferred carrying fluid, particularly advantageous when organic residues are problematic as is often the case in the microelectronic industry.
Other carrying fluids can also be used. These include, but are not limited to, polar organic solvents, such as alcohols and ketones, and mixtures of water and polar organic solvents.
In some embodiments, especially those using non-colloidal magnetic particles, viscosity additives may be required. These can be chosen from water soluble polar polymeric materials, such as polyalcohols, having molecular weights (MW) of about 300,000 to about 1,000,000. A preferable viscosity additive is polyethylene glycol (PEG). A typical static viscosity for the MPFs of this invention, that is the viscosity of the composition when not being used for processing, is about 300-1200 cps.
KOH, NaOH, and tetramethylammonium hydroxide, preferably the latter, and acidic compounds such as acetic acid and other weak organic acids can be used to adjust pH. The pH of the magnetic fluids should be greater than 9 to about 12.
The exact pH of an MPF composition depends on its intended use. Polishing silica wafers, for example, requires a pH in the 10.5-11 range, while polishing different types of glasses requires pHs of 11.8-12.0.
The MPFs of the present invention can be prepared in several ways, the exact method of preparation depending on the composition of the fluid. A preferable method for preparing the MPFs of this invention is a multi-step process.
When non-colloidal size magnetic particles are used, a dispersion of such particles in a carrying fluid is prepared. The powder is usually dispersed in the presence of a viscosity additive, after the additive has been mixed into the carrying fluid. When the carrying fluid itself is a multi-component system, the carrying fluid is usually prepared from its components prior to adding the viscosity additive.
The method for preparing a colloidal magnetic particle dispersion is similar to that of the non-colloidal one discussed immediately above except that a stabilizer replaces the viscosity additive. The colloidal magnetic powder can be purchased or synthesized. When the colloidal powder is synthesized, the stabilizing agent is usually added directly to the powder immediately after synthesis. Colloidal size iron particles can be synthesized as described in Example II below or by other methods described in the literature.
A dispersion of stabilized colloidal size polishing particles in a carrying fluid is then prepared. The stabilized colloidal dispersion can be prepared by any known method. For example, if SiO2 is used as the polishing agent, a colloidal dispersion can be prepared by dialysis, reaction of sodium silicate with acid, or ion exchange. Commercially available colloidal polishing particle dispersions can also be used.
After the colloidal polishing particle dispersion and either the non-colloidal magnetic particle or the colloidal magnetic particle dispersion have been prepared, the magnetic particle dispersion and the polishing dispersion are gradually mixed together while stirring vigorously. The pH of the final mixture is then adjusted to a value greater than 9 to about 12.
The magnetic particle, polishing particle and stabilizer contents in the final MPF composition range from about 20 to about 55 weight percent (wt. %), from about 1 to about 20 wt. %, and from about 0.5 to about 25 wt. %, preferably from about 25 to about 35 wt %, from about 3 to about 15 wt. %, and from about 1 to about 8 wt. %, respectively. When a viscosity additive is required, it is present in the final MPF composition in a wt % varying from about 0.1 to about 10 wt %, preferably from about 1 to about 5 wt %. Generally, the larger the weight percent of stabilizers used, the slower the rate of polishing. It is to be understood that when the weight percent ranges shown above and elsewhere in this specification do not total 100%, the remainder is made up of carrying fluid, adjusted to the proper pH as described above.
A multi-phase magnetic polishing fluid can be prepared as follows. Separate dispersions of non-colloidal magnetic particles, colloidal magnetic particles and colloidal polishing particles are prepared as described above. After the three dispersions have been prepared, the stabilized colloidal polishing particle dispersion is slowly added to, and mixed vigorously with, the stabilized colloidal magnetic particle dispersion to form an intermediate dispersion. The intermediate dispersion is gradually mixed with the non-colloidal magnetic particle dispersion, while stirring vigorously. The pH of the final product is then adjusted to a pH greater than 9 to about 12.
The proportions of non-colloidal magnetic, colloidal magnetic and colloidal polishing particles in the final MPF composition range between 21-38.5, 3-7.5 and 1-10 wt %, preferably 25-28, 3.75-5.25 and 3-5 wt %, respectively. The proportions of the stabilizer and viscosity additive range between about 0.5 to about 10 wt. % and about 0.1 to about 5 wt. %, preferably from about 0.5 to about 5 wt % and from about 1 to about 5 wt. %, respectively. Generally, the ratio of non-colloidal to colloidal magnetic particles should be from about 4.9:1 to about 6.1:1, preferably from about 5.3:1 to about 5.7:1.
In the multi-phase embodiment, a method of preparation was described wherein the colloidal polishing particle dispersion is first mixed with a colloidal magnetic particle dispersion. The resulting intermediate dispersion is then further mixed with the non-colloidal magnetic particle dispersion. An alternate, but equally acceptable, procedure would be to first mix the abrasive and non-colloidal magnetic dispersions to form an intermediate dispersion which in turn would be mixed with the colloidal magnetic dispersion.
The above MPFs can contain optional additives for increasing the rate and enhancing the quality of the polishing process. Examples of such additives are ammonium molybdate and zirconium sulfate, where the former serves as a chelating agent for abraded Si(OH)4 while the latter serves to increase the polishing rate. Small amounts of cerium hydroxide are often added to improve polishing quality. Zinc salts are optional additives that act as a precipitant for removing attrited silica. The above additives are added in amounts usually ranging from about 0.5 to about 4.5 wt. %, depending on the additive being used and the substrate being polished. These additives are generally added to the colloidal polishing particle dispersion immediately after it is prepared.
The polishing fluids of the present invention can be used with most of the magnetic polishing machines discussed in the literature. Typical magnetic polishing machines are described in U.S. Pat. Nos. 4,821,466, 5,449,313, 5,577,948, 5,616,066, and 5,839,944. In addition, the compositions of the present invention were tested using a polishing machine that employs a series of intermittent impacts from a plasticized quasi-solid MPF.
The polishing fluids of the present invention can be used to polish ceramic wafers for the microelectronics industry, optical materials, superconductor lamina, ceramic aircraft bearings, and materials for microelectromechanical systems (MEMS).
The following are several magnetic polishing fluid compositions prepared using the procedures described above.